“Virtual substrates” based on silicon (Si) and germanium (Ge) provide a platform for new generations of very large scale integration (VLSI) devices that exhibit enhanced performance in comparison to devices fabricated on bulk Si substrates. The important component of a SiGe virtual substrate is a layer of SiGe that has been relaxed to its equilibrium lattice constant (i.e., one that is larger than that of Si). This relaxed SiGe layer can be directly applied to a Si substrate (e.g., by wafer bonding or direct epitaxy), or atop a relaxed graded SiGe buffer layer in which the lattice constant of the SiGe material has been increased gradually over the thickness of the layer. The SiGe virtual substrate may also incorporate buried insulating layers, in the manner of a silicon-on-insulator (SOI) wafer. To fabricate high-performance devices on these platforms, thin strained layers of semiconductors, such as Si, Ge, or SiGe, are grown on the relaxed SiGe virtual substrates. The resulting biaxial tensile or compressive strain alters the carrier mobilities in the layers, enabling the fabrication of high-speed and/or low-power-consumption devices.
One technique suitable for fabricating strained Si wafers can include the following steps:                1. Providing a silicon substrate that has been edge polished;        2. Epitaxially depositing a relaxed graded SiGe buffer layer to a final Ge composition on the silicon substrate;        3. Epitaxially depositing a relaxed Si1−xGex cap layer having a constant composition on the graded SiGe buffer layer;        4. Planarizing the Si1−xGex cap layer by, e.g., chemical mechanical polishing (CMP);        5. Epitaxially depositing a relaxed Si1−xGex regrowth layer having a constant composition on the planarized surface of the Si1−xGex cap layer; and        6. Epitaxially depositing a strained silicon layer on the Si1−xGex regrowth layer.        
The deposition of the relaxed graded SiGe buffer layer enables engineering of the in-plane lattice constant of the SiGe cap layer (and therefore the amount of strain in the strained silicon layer), while reducing the introduction of dislocations. The lattice constant of SiGe is larger than that of Si, and is a direct function of the amount of Ge in the SiGe alloy. As the SiGe graded buffer layer is epitaxially deposited, it will initially be strained to match the in-plane lattice constant of the underlying silicon substrate. However, above a certain critical thickness, the SiGe graded buffer layer will relax to its inherently larger lattice constant.
The process of relaxation occurs through the formation of misfit dislocations at the interface between two lattice-mismatched layers, e.g., a Si substrate and a SiGe epitaxial layer (epilayer). Because dislocations cannot terminate inside a crystal, misfit dislocations have vertical dislocation segments at each end (termed “threading dislocations”), that may rise through the crystal to reach a top surface of the wafer. Both misfit and threading dislocations have stress fields associated with them. As explained by Eugene Fitzgerald et al., Journal of Vacuum Science and Technology B, Vol. 10, No. 4, 1992, incorporated herein by reference, the stress field associated with the network of misfit dislocations affects the localized epitaxial growth rate at the surface of the crystal. This variation in growth rates may result in a surface cross-hatch on lattice-mismatched, relaxed graded SiGe buffer layers grown on Si.
The stress field associated with misfit dislocations may also cause dislocation pile-ups under certain conditions. Dislocation pile-ups are a linear agglomeration of threading dislocations. Because pile-ups represent a high localized density of threading dislocations, they may render devices formed in that region unusable. Inhibiting the formation of dislocation pile-ups is, therefore, desirable.
Dislocation pile-ups are formed as follows. (See, e.g., Srikanth Samavedam et al., Journal of Applied Physics, Vol. 81, No. 7, 1997, incorporated herein by reference.) A high density of misfit dislocations in a particular region of a crystal will result in that region having a high localized stress field. This stress field may have two effects. First, this stress field may present a barrier to the motion of other threading dislocations attempting to glide past the misfits. This pinning or trapping of threading dislocations due to the high stress field of other misfit dislocations is known as work hardening. Second, the high stress field may strongly reduce the local epitaxial growth rate in that region, resulting in a deeper trough in the surface morphology in comparison to the rest of the surface cross-hatch. This deep trough in the surface morphology may also pin threading dislocations attempting to glide past the region of high misfit dislocations. This cycle may perpetuate itself and result in a linear region with a high density of trapped threading dislocations, i.e., dislocation pile-up.
Numerous theories attempt to explain the nucleation of misfit dislocations regarding where they are formed in the crystal and by what process. These theories include: formation at pre-existing substrate dislocations; heterogeneous formation at defects; and homogeneous formation, i.e., formation in defect-free, perfect crystal regions. As explained by Eugene Fitzgerald, Materials Science Reports, Vol. 7, No. 3, 1991, the activation energy for homogeneous dislocation formation is so high that it is unlikely to occur. The most likely source of misfit dislocations in the crystal is heterogeneous nucleation at defects.
As discussed by Petra Feichtinger et al., Journal of the Electrochemical Society, Vol. 148, No. 7, 2001, the substrate edge may provide a significant source of defects that serve as heterogeneous nucleation sites for misfit dislocations. As cut from a boule, the substrate edge may need to be further shaped to enable, for example, the substrate to withstand further mechanical handling. This shaping may be done by an edge grinding operation in which grind wheels that mirror the desired edge contour mechanically remove substrate material along the substrate edge. Various grinding grain sizes may be used, depending on the final edge contour or roughness specifications. The choice of mechanical edge shaping process, as well any additional damage removal steps, strongly influence the extent to which the substrate edge may preferentially serve as a heterogeneous source for misfit dislocation nucleation. The mechanical edge shaping process is also commonly referred to as the “edge contour grinding process” or simply the “grinding process.” Examples of additional damage removal steps include, but are not limited to, no additional steps (i.e., edge is left unpolished), a caustic etch step, or a caustic etch step plus a chemical-mechanical edge polish step. The additional damage removal process is also commonly referred to as the “edge polish process” or simply the “polish process.”
In Feichtinger et al., the authors investigated misfit nucleation formation for p on p+ epitaxial Si, and were interested in preventing the formation of misfit dislocations. Therefore, they preferred a substrate edge finish process that minimized edge defects, and thus minimized the number of heterogeneous misfit nucleation sources.